With recent growing interest in energy storage technologies, research and development efforts on batteries are becoming more focused. Under these circumstances, electrochemical devices are attracting the most attention, and from among such devices, interest in the development of rechargeable secondary batteries is paramount.
As the demand for portable electronic devices significantly increases, so does that for secondary batteries. Of such secondary batteries, there are remarkable advances being made for lithium secondary batteries that have high energy density, high discharge voltage, and excellent output stability.
The making of highly functional and miniaturized electronic devices is giving rise to requirements for secondary batteries to also have high performance, miniature size, and be available in various forms. For example, since the thickness of a notebook computer largely depends on the size of its secondary battery, various research efforts are underway to reduce the thickness of secondary batteries, improve their capacity and performance, and alter their shapes. In addition, as the environment becomes a more serious concern, the development of electric vehicles and hybrid electric vehicles that employ secondary batteries is accelerating to address the issue of global warming.
A lithium secondary battery includes electrolyte and an electrode assembly immersed in the electrolyte. The electrode assembly includes: a positive electrode coated with a positive electrode mixture including a lithium transition metal oxide as an active material; a negative electrode coated with a negative electrode mixture including a carbon-based active material; and a separator. Since lithium ions migrate through the electrolyte, if the electrolyte leaks, the lithium transition metal may be exposed to air to cause the battery to explode. Also, a chemical reaction from overcharging of the battery may increase the inner pressure of the battery case, causing the battery to explode. To address these issues, lithium ion batteries require a protective circuit. As such, since lithium ion batteries have the possibility of explosion, commercialized lithium ion batteries are provided in the form of a pack with a protective circuit.
Safety is the most important factor for lithium or lithium ion batteries that use non-aqueous electrolyte. Of particular importance is the prevention of short circuiting and over-charging.
Such secondary batteries are classified into nickel cadmium batteries, nickel hydrogen batteries, lithium ion batteries, and lithium polymer batteries according to the materials used for their positive electrode, negative electrode, and electrolyte, and the batteries are further classified into cylinder type batteries, prismatic type batteries, and pouch type batteries according to their shapes.
With respect to battery shapes, prismatic type batteries and pouch type batteries, which are thin and can be used in products such as portable phones, are in high demand. In terms of materials, lithium ion batteries such as lithium cobalt polymer batteries, which have high energy density, high discharge voltage, and excellent safety, are popular.
A main area of research on secondary batteries is improving safety. When a lithium secondary battery operates under abnormal conditions such as with an internal short circuit, in a charge state in which current and voltage exceed allowable thresholds, when exposure to high temperature, or subjected to an impact from being dropped, the inner temperature and pressure of the battery may increase, causing the battery to explode.
FIG. 1 is a schematic view illustrating a typically configured pouch type polymer secondary battery 100 in the related art. Referring to FIG. 1, the pouch type polymer secondary battery 100 includes a pouch type battery case 10 and an electrode assembly 50. The case 10 includes an aluminum laminated sheet with an upper sheet 10a and a lower sheet 10b. The case 10 accommodates the electrode assembly 50 including a positive electrode, a negative electrode, and a separator (not shown) disposed therebetween, and is sealed such that electrode leads 22 and 32 connected to a positive electrode tab 21 and a negative electrode tab 31 of the electrode assembly 50 are exposed to the outside of the case 10.
In a typical pouch type polymer secondary battery as illustrated in FIG. 1, a positive electrode, a separator, and a negative electrode are stacked, and each of the positive and negative electrodes includes a non-coating portion (tab). In this case, a positive electrode tab of each layer of an electrode assembly is connected to a positive electrode lead, and a negative electrode tab of each layer is connected to a negative electrode lead, thereby reducing resistance.
FIGS. 2, 3 and 4 are schematic views illustrating a prismatic type battery having a jelly-roll shape in the related art. A positive electrode 20 and a negative electrode 30 are wound with a separator 40 therebetween. At least one of both surfaces of a positive electrode collector 23 constituting the positive electrode 20 is coated with a positive electrode active material 24. At least one of an active material coating start portion and an active material coating end portion of the positive electrode collector 23 is provided with a non-coating portion 25 that is not coated with the positive electrode active material 24. In FIG. 2, the non-coating portions 25 are disposed at both the active material coating start portion and the active material coating end portion.
At least one of both surfaces of a negative electrode collector 33 constituting the negative electrode 30 is coated with a negative electrode active material 34. At least one of an active material coating start portion and an active material coating end portion of the negative electrode collector 33 is provided with a non-coating portion 35 that is not coated with the negative electrode active material 34. In FIG. 2, the non-coating portion 35 is disposed at both the active material coating start portion and the active material coating end portion of the negative electrode collector 33.
The positive electrode active material 24 applied on the positive electrode collector 23 and the negative electrode active material 34 applied on the negative electrode collector 33 have predetermined widths, and the positive electrode 20 and the negative electrode 30 are wound in a jelly-roll shape to form the battery. The width of the negative electrode active material 34 may be greater than the width of the positive electrode active material 24.
The negative electrode 30 and the positive electrode 20 are provided with terminals that are externally connected. As illustrated in FIG. 2, a positive electrode tab 22 is disposed on the non-coating portion 25 that is a start portion not coated with the positive electrode active material 24, and a negative electrode tab 32 is disposed on the non-coating portion 35 that is an end portion not coated with the negative electrode active material 34.
Thus, when the positive electrode 20, the negative electrode 30, and the separator 40 disposed therebetween are wound, the non-coating portion 25 is disposed at a roll start portion not coated with the positive electrode active material 24, and the non-coating portion 35 is disposed at a roll end portion not coated with the negative electrode active material 34. Accordingly, an electrode assembly wound in a jelly-roll shape is connected to the positive electrode tab 22 and the negative electrode tab 32 as external terminals. As illustrated in FIG. 3, the positive electrode tab 22 is connected to the non-coating portion 25 in the innermost portion of the jelly-roll shape, and the negative electrode tab 32 is connected to the non-coating portion 35 in the outermost portion of the jelly-roll shape.
The electrode assembly configured as described above is accommodated in the prismatic type battery as illustrated in FIG. 4.
As described above, since the positive and negative electrode tabs 22 and 32 are disposed in the start portion and the end portion, respectively, the distance over which electric current should flow is increased, and thus, resistance is also increased. Therefore, a battery configured as described above may be inappropriate for a power tool, an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV), which require high output power.
Also, in a typical prismatic type battery, an electrode assembly including a positive electrode, a separator, and a negative electrode is not integrated. If an electrode assembly of a typical prismatic type battery is integrated, a subsequent process such as a tab lead welding process and a process of inserting an electrode assembly into an exterior part, can be facilitated, twisting of the electrode assembly due to contraction and expansion of an active material during charging and discharging can be suppressed, and a short circuit due to external shock can be prevented, thereby improving safety of the battery.
Thus, a high-power secondary battery, which includes an electrode assembly with a positive electrode and a negative electrode integrated through a separator and has improved processability and safety, is required.